The present invention relates to lasers and, more particularly, to RF excited metal waveguide lasers.
Recently, there has been considerable interest in waveguide gas lasers wherein the laser light propagates through a hollow waveguide which also serves to confine the laser-exciting discharge. Early forms of waveguide gas lasers are disclosed in the patent to Smith No. (3,772,661). The basic laser excitation scheme employed by Smith and used in most of the early waveguide gas lasers involved establishing a d.c. electric discharge longitudinally through a device between a pair of electrodes disposed near the respective ends of the laser waveguide. This type of discharge requires relatively large d.c. excitation voltages (i.e. about 10 kv) along with the necessary power supply and associated circuitry for generating voltages of such magnitude.
The aforementioned Smith patent also briefly discloses exciting a ring-type waveguide laser from an r.f. source by means of a coil wound about the ring-shaped waveguide. Such a coil excitation arrangement not only is incapable of providing a highly uniform discharge; but, it also affords poor coupling efficiency. Moreover, when more than a few turns are employed, the inductance of the coil itself becomes sufficiently large to limit the usable excitation frequency to below a few MHz.
In order to obtain a more uniform discharge with reduced excitation voltage, waveguide gas lasers have been developed wherein a pulsed discharge is established along a transverse waveguide dimension. Such lasers are shown in the patent to Smith et al. No. (3,815,047). Waveguide lasers of that type have been operated in a quasi-continuous mode at pulse repetition rates as high as 40 kHz, as described in the Smith et al. paper "High Repetition Rate and Quasi-CW Operation of a Waveguide CO.sub.2 TE laser", Optics Communications, Volume 16, no. 1 (January 1976), pp. 50-53.
In lasers employing longitudinal and transverse electric discharges according to the above-described teachings, the cathodes are usually sufficiently poor electron emitters so that positive ion current dominates in the region immediately adjacent to the cathode. As a result, a positive space charge is formed in this region. The electric fields resulting from the positive space charge cause electrons emitted from the cathode to be accelerated sufficiently so that an avalanche ionization effect occurs in the space charge region. By the outer extremity of this space charge region, the electron density is sufficiently large so that an electron dominated current occurs throughout the remainder of the discharge. In the space charge region, the discharge voltage increases very rapidly in a positive direction as a function of distance from the cathode. This is typically by about 400 to 600 volts in waveguide laser-exciting transverse discharges. As a consequence, the space charge region is often referred to as the "cathode fall" region. Throughout the remainder of the discharge, i.e., between the cathode fall region and the anode, the discharge voltage increases very slowly in a positive direction as a function of distance from the cathode.
These and other effects in the cathode fall region give rise to a number of problems in previous discharge-excited waveguide lasers. First, positive ion bombardment of the cathode has a tendency to damage the cathode, thereby limiting the life of the device. Also, the high electric fields in the cathode fall region tend to dissociate the laser gas. In addition, the relatively large cathode fall voltage wastes a substantial amount of input energy, thereby reducing operating efficiency. Further, considerable additional circuit hardware such as high voltage power supplies, current regulators, and ballast resisters may be required to provide the relatively large excitation voltages, as well as to overcome instabilities resulting from negative impedence discharges. Moreover, in pulsed transverse discharge excitation in lasers according to the prior art, the excitation pulse duration must be sufficiently short to preclude arcing, and bulky and expensive post-formation networks are required.
An improvement to the above-described lasers is shown in the patent to Laakmann No. (4,169,251). In a waveguide laser according to Laakmann, a laser gas is disposed in an elongated chamber of cross-sectional dimensions suitable for guiding laser light. A laser-exciting discharge is established in the laser gas by means of an alternating electric field applied to the chamber along a direction transverse to its length. The excitation frequency probably ranges from a value of about V/2d to about 50V/d, where d is the distance across the chamber and V is the drift velocity of electrons in the laser gas, having a value ranging typically from about 5.times.10.sup.6 cm per second to about 1.5.times.10.sup.7 cm per second. For typical laser gases and operating pressures, appropriate excitation frequencies generally lie in the VHF-UHF range, i.e., from about 30 MHz to about 3 GHz. These excitation frequencies are sufficiently high to ensure negligible interaction of discharge electrons with the electric field-applying electrodes, thereby tending to provide a discharge-excited waveguide laser which has increased operating life, reduced tendency towards laser gas dissociation, increased discharge stability and uniformity, increased operating efficiency, a significant lowering of required excitation voltages, and a substantial reduction in excitation hardware as compared to the then prior art laser devices.
A simplified drawing of a laser according to the teachings of Laakmann, generally indicated as 10, is shown in FIGS. 1 and 2, which generally correspond to FIGS. 2 and 1 respectively, of the Laakmann patent. As can be seen therein, the laser 10 has a pair of electrodes 12 and 14 which are elongated and of rectangular cross-section. The electrodes 12, 14, are disposed in parallel spaced relationship to form a discharge region 16 therebetween. The lower electrode 14 is attached to a large block 18 of copper, or the like, which both supports the electrode 14 and, more importantly, aids in cooling the electrode 14 by conduction. A pair of elongated, rectangular, ceramic blocks 20 are attached to the block 18 and electrode 14. The upper electrode 12 is also attached to the ceramic blocks 20. A pair of window members 22 are attached over the ends of the rectangular waveguide conduit 24 formed by the electrodes 12, 14 and blocks 20. The electrodes 12, 14, block 18, and window members 22 are suitably sealed to one another so that no leakage occurs with respect to the waveguide conduit 24. Laser gas 26 is placed within the conduit space 24, and the excitation circuit, generally indicated as 28, is used to excite the electrodes 12, 14 in the manner described above.
These standard, prior art, r.f. waveguide lasers, as typified by the Laakmann laser of FIGS. 1 and 2, have a rectangular cross-section in which two of the sides are the metal (typically aluminum) r.f. electrodes 12, 14, and the remaining two sides are a dielectric material (i.e. the ceramic blocks 20), the principal function of which is to confine the laser radiation into a rectangular waveguide mode. Although this design has many desirable features, as proved by its commercial success, the need for dielectric waveguide walls is a constantly remaining undesirable feature. Suitable dielectric components are costly and difficult to fabricate, and they may also limit the laser's performance and lifetime. The quantity of available laser gas is small and, consequently, among other things, the cooling of the electrode 12 by convection and conduction is poor at best, resulting in deterioration of the sidewalls of the waveguide conduit 24 with attendant shortening of life and performance of the laser.
Wherefore, it is the object of the present invention to provide an r.f. excited metal waveguide laser eliminating the dielectric sidewalls of the waveguide region and, additionally, providing a higher volume of laser gas and more efficient cooling of the components.